1. Field of the Invention
This invention relates to improvements in circuitry for driving polyphase dc motors, and still more particularly to methods and apparatus for detecting reverse rotation of a rotor of a polyphase dc motor using information concerning the crossing of a reference voltage by the back emf of non-selected field coils.
2. Description of the Prior Art
Although the present invention pertains to polyphase dc motors, in general, it finds particular application in conjunction with three phase dc motors, particularly of the brushless, sensorless type that are used for rotating data media, such as found in computer related applications, including hard disk drives, CD ROM drives, floppy disks, VCRs, and the like. Such three phase brushless, sensorless dc motors are becoming more popular in computer applications due to their reliability, low weight, and accuracy.
Motors of this type can typically be thought of as having a stator with three coils connected in a "Y" configuration, although actually, a larger number of stator coils are usually employed with multiple motor poles. In such applications, the "Y" connected stator coils are typically connected in three sets of four coils, each physically separated by 90.degree., with eight pole rotors that have four electrical cycles per rotor revolution. In bipolar applications, the coils are energized in sequences. In each sequence, a current path is established through two coils of the "Y", with the third coil left floating. The sequences are designed so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. Moreover, the sequence is designed so that when the floating coil is switched into the current path, current will continue to flow in the same direction in the coil which was included in the prior current path. In this manner, six commutation sequences are defined for each electrical cycle of the three phase motor.
During the operation of such a motor, it has been recognized that maintaining a known position of the rotor is an important concern. There have been various ways by which this was implemented. The most widely used way, for example, was to start the motor in a known position, then develop information related to the instantaneous or current position of the rotor. One source of such instantaneous position information can be developed as a part of the commutation process by identifying the floating coil, and monitoring its back emf, that is, the emf induced into the coil as it moves through the magnetic field provided by the stator. When the voltage of the floating coil crosses zero (referred to in the art as "a zero crossing"), or a predetermined reference voltage, the position of the rotor can be determined. Upon the occurrence of this event, the rotor coil commutation sequence can be incremented to the next phase, and the process repeated. The assumption that the zero crossing accurately indicates the rotor position is generally true if the motor is functioning properly, and nothing has occurred which would disturb its synchronization from its known startup position. However, events may occur which nevertheless result in a loss of synchronization.
The possibility of loss of synchronization made the motors previously used vulnerable and delicate, and great care had to be taken to insure that the startup algorithms and running conditions were precisely controlled to avoid anything which might cause such out of synchronization condition to occur.
Moreover, one of the problems associated with such a motor is that generally after operation the position at which the rotor has stopped is not known, and to restart the motor by the application of startup voltage in a random fashion may tend to initially start the motor in the wrong direction. This can be a major problem in magnetic disk applications where backward rotation may damage the read/write heads.